Root-infecting, nonpathogenic fungi capable of promoting plant growth have been demonstrated, the classic examples being various mycorrhizae (Garrett, S. D. [1970] Pathogenic Root-Infecting Fungi, University Press, p. 294; Harley, J. L., and S. E. Smith [1983] Mycorrhizal Symbiosis, Academic Press, London). Mycorrhizae are known to increase photosynthetic and water use efficiency and nutrient uptake under nutrient-deficient soils (Graham, J. H. et al. [1987] New Phytol. 105:411-419; Nemec, S. and G. Guy [1982] J. Am. Soc. Hort. Sci. 107:177-180). They are also known to produce auxins (Slankis, V. [1958]; Moser [1959]) and cytokinins (Miller, C. O. [1971] In Mycorrhizae, E. Hacskaylo, ed.).
Pathogenic fungi causing growth promotion in plants are also known. In fact, gibberellin, one of the major growth promoting hormones in plants, was first isolated from Gibberella fujikuroi, a fungus which infects rice plants (Cross, B. E. [1954] J. Chem Soc. 4670-4676). Rademacher and Graebe (Rademacher, W., and J. E. Graebe [1979] Biochem. Biophys. Res. Commun. 91:35-40) reported the production of gibberellin A.sub.4 by Sphaeclona manihoticola which is a fungal pathogen of cassava.